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Network Working Group                                         W. SimpsonRequest for Comments: 1788                                    DaydreamerCategory: Experimental                                        April 1995ICMP Domain Name MessagesStatus of this Memo   This document defines an Experimental Protocol for the Internet   community.  This does not specify an Internet standard of any kind.   Discussion and suggestions for improvement are requested.   Distribution of this memo is unlimited.IESG Note:   An Internet Engineering Steering Group comment from the co-Area   Director for IPng:  Please note well that this memo is an individual   product of the author.  It presents one view of the IN-ADDR   mechanism, motivated by discussion in the IPNG WG of the difficulty   of secure, dynamic update of the reverse tree.  Other IETF discussion   and ongoing standards work on this area will be found in the IP Next   Generation (ipngwg), DNS IXFR, Notification, and Dynamic Update   (dnsind), DNS Security (dnssec) working groups.Abstract   This document specifies ICMP messages for learning the Fully   Qualified Domain Name associated with an IP address.Simpson                                                         [Page 1]

RFC 1788                    ICMP Domain Name                  April 1995Table of Contents1.     Introduction ..........................................21.1       Direct Query ....................................31.2       Multicast .......................................31.3       Domain Names ....................................31.4       Messages ........................................42.     Domain Name Request ...................................43.     Domain Name Reply .....................................5     SECURITY CONSIDERATIONS ......................................6     REFERENCES ...................................................6     ACKNOWLEDGEMENTS .............................................7     AUTHOR'S ADDRESS .............................................71.  Introduction   The Domain Name System (DNS) is described in [RFC-1034].  The IN-ADDR   domain of the DNS is specified [RFC-1035] to perform address to   domain name resolution, and to facilitate queries to locate all   gateways (routers) on a particular network in the Internet.   Neither function has been remarkably successful.  The IN-ADDR domain   is not reliably populated.   As multiple routers were used at boundaries and within networks, the   IN-ADDR mechanism was found to be inadequate.  The location of   routers by hosts is now performed using "ICMP Router Discovery   Messages" [RFC-1256].   As network numbers migrated to "classless" routing and aggregation,   the IN-ADDR delegation granularity has fragmented, and requires   overlapping administration.  The "reverse" IN-ADDR administration   frequently does not follow the same delegation as the "forward"   domain name tree.  This structure is not amenable to cooperative   secure updating of the DNS.   As application servers have appeared which require the Domain Name   for user interaction and security logging, the IN-ADDR servers have   been inundated with queries.  This produces long user visible pauses   at the initiation of sessions.Simpson                                                         [Page 2]

RFC 1788                    ICMP Domain Name                  April 19951.1.  Direct Query   This document proposes that each unicast address be queried directly   for its corresponding Domain Name.  This has the advantages that the   naming is under the same administration as the address assignment,   and the queries are distributed in the same fashion as IP routing.   In effect, the routing is used to index the database.1.2.  Multicast   Only a few well-known multicast addresses are populated in the IN-   ADDR domain.  The ephemeral nature of most multicast addresses is not   conducive to cooperative secure updating of the DNS.   However, the technique described here is not useful for multicast   addresses.  A query to a multicast address could result in a storm of   replies.  Most multicast groups are not named, or the member nodes   are not configured with the name.   The IN-ADDR method SHOULD continue to be used for reverse lookup of   well-known multicast addresses in the range 224.0.0.0 to   224.0.255.255.  Other multicast addresses are an issue for futher   study.1.3.  Domain Names   Each Domain Name is expressed as a sequence of labels.  Each label is   represented as a one octet length field, followed by that number of   octets.  Since every Domain Name ends with the null label of the   root, a Domain Name is terminated by a length byte of zero.  The most   significant two bits of every length octet must be '00', and the   remaining six bits of the length field limit the label to 63 octets   or less.   When the most significant two bits of the length octet are '11', the   length is interpreted as a 2 octet sequence, indicating an offset   from the beginning of the message (Type field).  Further details are   described in [RFC-1035] "Message Compression".   To simplify implementations, the total length of a Domain Name   (including label octets and label length octets) is restricted to 255   octets or less.Simpson                                                         [Page 3]

RFC 1788                    ICMP Domain Name                  April 19951.4.  Messages   The datagram format and basic facilities are already defined for ICMP   [RFC-792].   Up-to-date values of the ICMP Type field are specified in the most   recent "Assigned Numbers" [RFC-1700].  This document concerns the   following values:       37  Domain Name Request       38  Domain Name Reply2.  Domain Name Request   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |     Code      |          Checksum             |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |           Identifier          |        Sequence Number        |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Type             37   Code             0   Checksum         The ICMP Checksum.   Identifier       If Code is zero, a value to aid in matching requests                    and replies.  For example, it might be used like a                    port in TCP or UDP to identify a session.  May be                    zero.   Sequence Number  If Code is zero, a value to aid in matching requests                    and replies.  For example, the number might be                    incremented on each request sent.  May be zero.   A separate Domain Name Request is used for each IP Destination   queried.   An ICMP Domain Name Request received with a broadcast or multicast   Destination MUST be silently discarded.   On receipt of an ICMP error message, the implementations MAY attempt   to resolve the Domain Name using the IN-ADDR method.Simpson                                                         [Page 4]

RFC 1788                    ICMP Domain Name                  April 19953.  Domain Name Reply   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |     Code      |          Checksum             |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |           Identifier          |        Sequence Number        |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                          Time-To-Live                         |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |   Names ...   +-+-+-+-+-+-+-+-   Type             38   Code             0   Checksum         The ICMP Checksum.   Identifier       Copied from the request.   Sequence Number  Copied from the request.   Time-To-Live     The number of seconds that the name may be cached.                    For historic reasons, this value is a signed 2s-                    complement number.   Names            zero or more Fully Qualified Domain Names.  The                    length of this field is determined from the total                    length of the datagram.                    When no names are known, the field is eliminated                    (zero length), but the Reply is sent as an                    authoritative indication that no name is known.                    When more than one name is known, all such names                    SHOULD be listed.                    Any name which cannot entirely fit within the Reply                    MTU is not sent.   The IP Source in a Reply MUST be the same as the IP Destination of   the corresponding Request message.   Every host and router MUST implement an ICMP Domain Name server   function that receives Domain Name Requests and sends corresponding   Domain Name Replies.Simpson                                                         [Page 5]

RFC 1788                    ICMP Domain Name                  April 1995   A host SHOULD also implement an application- layer interface for   sending a Domain Name Request and receiving a Domain Name Reply, for   diagnostic purposes.Security Considerations   A primary purpose of this specification is to provide a mechanism for   address to name resolution which is more secure than the IN-ADDR   reverse tree.  This mechanism is amenable to use of the IP Security   Protocols for authentication and privacy.   Although the routing infrastructure to the Destination does not   provide security in and of itself, it is as least as reliable as   delivery of correspondence for the other sessions with the same peer.   A DNS cryptographic signature, located by using the reply in the   forward DNS direction, can be used to verify the reply itself.References   [RFC-792]            Postel, J., "Internet Control Message Protocol", STD 5,RFC 792, USC/Information Sciences Institute, September            1981.   [RFC-1034]            Mockapetris, P., "Domain Names - Concepts and Facilities",            STD 13,RFC 1034, USC/Information Sciences Institute,            November 1987.   [RFC-1035]            Mockapetris, P., "Domain Names - Implementation and            Specification", STD 13,RFC 1035, USC/Information            Sciences Institute, November 1987.   [RFC-1256]            Deering, S., Editor, "ICMP Router Discovery Messages",RFC 1256, Xerox PARC, September 1991.   [RFC-1700]            Reynolds, J., and J. Postel, "ASSIGNED NUMBERS", STD 2,RFC 1700, USC/Information Sciences Institute, October 1994.Simpson                                                         [Page 6]

RFC 1788                    ICMP Domain Name                  April 1995Acknowledgements   The DNSIND and IPng Working Groups contributed substantial amounts of   discussion.   Additional comments should be submitted to the   namedroppers@internic.net mailing list.Author's Address   Questions about this memo can also be directed to:      William Allen Simpson      Daydreamer      Computer Systems Consulting Services      1384 Fontaine      Madison Heights, Michigan  48071      Bill.Simpson@um.cc.umich.edu          bsimpson@MorningStar.comSimpson                                                         [Page 7]